Let's try some of the great C19th novelists. Take War and Peace, for instance. The focus is on Pierre and Natasha. Trollope has huge cast lists, but each novel is clearly focussed on one person, from Septimus Harding onwards. Ditto Dickens. Despite the great sweep of these novels we always know where the focus is.
I suspect it's because as people we're geared up to have intense relationships with only a few people. A large cast of characters without focus is like being at a drinks party where you talk to lots of people about superficial things; you simply can't get deep and meaningful with all of them. That's not to say the large cast shouldn't be colourful - it's best if they are - but that as a writer you should know where your main story lies.